1. Field of the Invention
The present invention relates to a wavelength tunable optical filter for inputting or outputting light with a desired wavelength in an optical communication system of a wavelength division multiplexing (WDM) scheme and, more particularly, to a wavelength tunable optical filter operated by a thermal actuation.
2. Description of the Related Art
An optical communication system using a wavelength division multiplexing (WDM) scheme sends/receives optical signals of various channels each of which has a different wavelength, through an optical fiber, and needs a process of combining or splitting light having a desired wavelength at each of terminal and node. For this process, a wavelength tunable filter is used. The prior art wavelength tunable filter is comprised so that the distance between distributed bragg reflectors (DBR's) is shortened by an electrostatic force, as a voltage is applied between the distributed bragg reflectors (DBR's) alternatively stacked and made of semiconductors or dielectric materials, which have a different refractive index. The wavelength tunable filter by an electrostatic force has the actuation range limited to less than ⅓ of an initial gap between the two mirrors, since two mirrors are stuck when the distance between the two mirrors is less than ⅔ of the initial gap. Also, in the case where the number of DBR pairs increases in order to raise wavelength selectivity, the thickness of the actuator is also increased. Thus, the actuation voltage is rapidly increased. In this case, since an additional high voltage generator is needed, the manufacturing cost is increased. If the actuation voltage exceeds a critical value, a discharge between the two mirrors occurs and an insulator is damaged, thereby the elements can be no longer used. Accordingly, in order to prevent the above problems, the methods for manufacturing the mirrors and the actuators with different material or different thickness have been studied. However, there is a disadvantage in that the methods are too much complicated.
Recently, actuation methods using thermal expansion have been studied in order to solve these problems.
One of them makes use of a principle that, when a cantilever joined up and down by two materials having a different thermal expansion coefficient is heated, bending occurs toward the material having a smaller thermal expansion coefficient (T. Amano et al., J. Light wave Technol., vol. 21, pp 596˜601, 2003). Namely, after manufacturing a cantilever type actuator joined up and down by two materials having a different thermal expansion coefficient, one side of the actuator is fixed to an anchor and the other side is connected to a mirror. The cantilever is bent in an upward or downward direction by heat generated when electrical current flows between two electrodes on the anchor, thereby the mirror connected to the end of the cantilever also moved such that the distance between the mirrors increases or decreases. Accordingly, the motion direction of the mirror can be determined by relatively enhancing the thermal expansion coefficient of the upper or the lower part of the cantilever structures in the initial manufacturing process. This method has a problem that, since the one side of the mirror is fixed to the cantilever, the two mirrors initially set to parallel to each other are sloped as the cantilever is curved, thereby the line width becomes wide and the transmissivity of light becomes low. Meanwhile, since the heat generated at the anchor as the electrical current flows is transferred to the cantilever, a part of heat is lost at the anchor and the remaining heat is used to actuate the mirror. Accordingly it has a disadvantage in that the efficiency is low and the power consumption is high.
The other one makes use of longitudinal thermal expansion of the actuator. Namely, four actuators supporting a mirror are connected to electrodes two by two, and then, when electrical current flows from one electrode to the other, heat is generated from the actuators, thereby the length of the actuator is increased and it is bent in a direction where the mirrors moves away from each other (F. Riemenschneider et al. IEEE Photon. Technol. Lett., vol. 14, pp. 1566˜1568, 2002). This structure is manufactured in such a manner that two substrates on which mirrors have been formed respectively are joined with the mirrors facing each other and then one of the substrate is removed from the other. At this time, to prevent the two mirrors from being attached each other, one of the mirrors is composed of two materials having a different intrinsic stress so that the mirror is naturally deformed in a convex form in the process of removing the substrate. However, since this method makes it difficult to constantly reproduce the radius of curvature of the mirror convexly transformed and the gap between the two substrates, it has a disadvantage in that initial gap between the two mirrors is not constant. Also, if the distance between the two mirrors is denoted by ‘d’ and the refractive index of the material existed between the two mirrors is denoted by ‘n’, the free spectral range (FSR) Δλ of adjacent resonant wavelengths can be defined as λ2/2nd. Since the initial distance, d, between the two mirrors becomes tens of μm, FSR is less than tens of nm. Accordingly, even though the actuation distance is over 100 nm, the available wavelength tuning range is limited to tens of nm by FSR. In addition, since the convex mirror is used, if a beam waist of the incident light is not precisely adjusted to a curvature of the curved mirror, then the transmission line width becomes wide. Therefore the wavelength selectivity is decreased, while the insertion loss is increased.